Conventionally, many propositions have been made with respect to the production of synthetic resin optical sheeting comprising synthetic resin sheeting provided on a surface thereof with an array of various optical elements as described above.
In the production of such synthetic resin optical sheeting, highly precise processing is required because the shape accuracy of the optical elements determines its optical performance, as contrasted with general processing techniques for synthetic resins, such as so-called embossing, graining and satinizing. For example, in the case of so-called microprism type retroreflective sheeting comprising synthetic resin sheeting provided with an array of cube corners, the angular divergence of the bundle of retroreflected light will become too great for practical purposes even if the three mutually perpendicular faces constituting each prism is deviated during forming, for example, by about 1.degree..
In order to overcome this disadvantage, various attempts have been made to improve the method of forming an array of optical elements on a surface of synthetic resin sheeting. Several patents disclosing techniques for the production of synthetic resin optical sheeting are described below.
In Rowland U.S. Pat. No. 3,689,346, there is described a method for continuously producing cube-corner type retroreflective sheeting. According to this method, retroreflective sheeting is produced by depositing a curable molding material upon a cube-corner mold and applying a transparent, flexible film material over the molding material, after which the molding material is cured and bonded to the film material. However, the resins specifically described in this patent for use as molding materials are substantially limited to crosslinkable resins such as plastisol type vinyl chloride resin containing a crosslinking acrylic ester monomer. Although it is suggested that some resins in a molten state can be used, no specific description is given of the melt viscosity conditions, heating conditions and cooling conditions under which such a resin can be embossed to obtain a product having high shape accuracy.
In Rowland U.S. Pat. No. 4,244,683 (corresponding to Japanese Patent Publication No. 51320/'81), there is disclosed an apparatus and method (i.e., a so-called progressive pressure forming method) for semi-continuously embossing thermoplastic synthetic resin sheeting to form cube-corner prisms on one surface thereof. In this patent, it is described that prism elements are formed by placing a series of flat embossing molds on one surface of synthetic resin sheeting traveling on an endless belt having a smooth surface, and pressure-forming the sheeting successively in three types of press stations (i.e., a preheating station, a thermoforming station, and a plurality of cooling stations). However, the prism sheeting produced by this method shows seams distinctly owing to the use of juxtaposed flat molds, resulting in a poor appearance of the product. Moreover, this method has the additional disadvantage of being inferior in productivity.
In Pricone et al. U.S. Pat. No. 4,486,363 (corresponding to Japanese Patent Laid-Open No. 140021/ '84) and Pricone et al. U.S. Pat. No. 4,601,861 (corresponding to Japanese Patent Laid-Open No. 47237/'86), there are disclosed apparatus and methods for continuously embossing cube-corner prisms on one surface of thermoplastic synthetic resin sheeting. According to the embossing methods described in these patents, a portion of an embossing tool comprising an endless belt having a precision embossing pattern is heated to a temperature above the glass transition temperature of the thermoplastic synthetic resin. Thereafter, the thermoplastic synthetic resin sheeting is continuously embossed at a plurality of pressure points and then cooled to a temperature below the glass transition temperature of the thermoplastic synthetic resin in a cooling station.
However, in the methods described in these patents, the embossing temperature is limited to a temperature higher than the glass transition temperature of the synthetic resin and lower than the glass transition temperature of a carrier film. Consequently, the resin tends to have insufficient fluidity, and hence requires a long pressing time or a plurality of pressure points. Thus, these methods are not considered to be highly productive. Moreover, these methods have the disadvantage that the elements embossed under such temperature conditions show a reduction in shape accuracy owing to elastic deformation. Furthermore, since the embossing temperature is limited to a temperature higher than the glass transition temperature of the synthetic resin and lower than the glass transition temperature of a carrier film as described above, the choice of a carrier film is disadvantageously limited when high-melting synthetic resin sheeting made, for example, of a polycarbonate resin is to be embossed.
In Japanese Patent Laid-Open Nos. 159039/'81 and 159127/'81, there is disclosed a method in which synthetic resin sheeting is fed between a pair of endless belts and an array of optical elements such as lenticular lenses or Fresnel lenses is formed on one surface of the synthetic resin sheeting. However, these patents give no specific description of the melt viscosity conditions, heating conditions and cooling conditions which can be employed for the embossing of the synthetic resin. Moreover, the apparatus disclosed therein does not have a construction which enables the embossed synthetic resin to be stripped after sufficient cooling, and is not suitable for the formation of an array of optical elements having satisfactorily high optical accuracy.
In Japanese Patent Laid-Open No. 107502/'92, it is described that hologram sheeting is formed by extruding synthetic resin sheeting in a molten state from an extruder and, between a pair of elastic rolls and a cooling roll, pressing the sheeting against an endless stamper mounted on the cooling roll and another roll to emboss a diffraction grating. However, the pressed synthetic resin sheeting is not sufficiently cooled in the method described therein, so that it is difficult to form optical elements having a highly accurate shape according to this method. Moreover, this patent gives no specific description of the melt viscosity conditions, heating conditions and cooling conditions under which synthetic resin sheeting can be embossed to obtain a product having good shape quality.
Next, the problems encountered when an array of optical elements (e.g., cube-corner prisms) as described above is continuously formed on synthetic resin sheeting according to the above-described prior art methods are described below.
According to experiments conducted by the present inventors, the problems associated with the formation of an array of optical elements, which are expected to arise in the above-described prior art methods, include insufficient filling into the mold due to poor fluidity of the synthetic resin, elastic deformation after pressure release due to insufficient melting, shrinkage due to chemical linkages of the synthetic resin and/or evaporation of the solvent and low-molecular components in the synthetic resin, and the like.
Insufficient filling into the mold due to poor fluidity of the synthetic resin tends to occur especially when a synthetic resin is fed to an embossing tool in the form of sheeting having a relatively low temperature (e.g., in the vicinity of room temperature), because the synthetic resin cannot be sufficiently heated before or in a thermoforming zone. Especially in the methods described in Pricone et al. U.S. Pat. No. 4,486,363 (corresponding to Japanese Patent Laid-Open No. 140021/'84) and Pricone et al. U.S. Pat. No. 4,601,861 (corresponding to Japanese Patent Laid-Open No. 47237/'86), the temperature at which synthetic resin sheeting is embossed is limited so as to be higher than the glass transition temperature of the synthetic resin and lower than the glass transition temperature of a carrier film, so that the synthetic resin tends to have insufficient fluidity.
For this reason, Pricone et al. have employed a method in which a plurality of pressure points are provided or a method in which the traveling speed of the synthetic resin sheeting is reduced to provide a sufficient pressing time. However, neither of these methods is considered to be highly productive. Moreover, when a plurality of pressure points are carelessly provided, undesirable misalignment of the pattern may result because the synthetic resin sheeting separated from the mold is pressure-formed again at the next pressure point. Furthermore, as another improvement, a method in which a synthetic resin melted, for example, in an extruder is fed in the form of sheeting is also described therein. However, no specific description is given of the temperature conditions and synthetic resin melting conditions which can be employed to form a highly accurate array of optical elements.
Elastic deformation after pressure release due to insufficient melting tends to occur when an insufficiently melted synthetic resin is forcibly filled into a mold under high pressure. In this case, the synthetic resin is completely filled into the mold, but elastic deformation after pressure release tends to produce defects such as the swelling of the optical element-bearing surface so as to be convex rather than planar. Moreover, high-pressure forming under such conditions also has the disadvantage of shortening the life of the mold.
Deformation of the optical elements due to shrinkage of the synthetic resin is a phenomenon in which, during pressure forming or after pressure release, the optical element-bearing surface required to be essentially planar shrinks and deforms into a concave surface owing to chemical linkages, evaporation of the solvent and low-molecular components in the synthetic resin, and the like. This shrinking phenomenon tends to prevent the synthetic resin from being stripped from the mold and thereby cause a reduction in productivity. Moreover, when low-molecular components are contained in the synthetic resin, these low-molecular components tend to adhere to the surface of the mold and cause such troubles as poor strippability and a reduction in surface smoothness. Furthermore, any volatile components remaining in the resulting embossed sheeting cause an additional trouble in that they gradually evaporate during use of the product to deform the optical elements and thereby detract from the performance of the product.
The present inventors have made investigations with a view to solving the above-described problems encountered when an array of optical elements is continuously formed on synthetic resin sheeting and, in particular, when an array of cube-corner type microprisms for use in the production of cube-corner type retroreflectors is continuously formed thereon. As a result, it has now been discovered that synthetic resin sheeting having an array of optical elements thereon and exhibiting high optical accuracy can be made with good productivity, by heating synthetic resin sheeting to a temperature in the flow temperature region of the synthetic resin, feeding the heated synthetic resin sheeting directly to a thermoforming zone of embossing means, continuously pressing the sheeting against a mold mounted on the embossing means and having a pattern for defining an array of optical elements while maintaining the sheeting at that flow temperature, to bring the sheeting into intimate contact with the mold and thereby form an array of optical elements on one surface of the sheeting, laminating a surfacing film to the side of the sheeting opposite to the mold as required, feeding a carrier film to the side of the resulting laminate opposite to the mold and bringing the carrier film into close contact therewith, and cooling the sheeting to a temperature lower than the glass transition temperature of the synthetic resin. The present invention has been completed on the basis of this discovery.